From my window in Hobart, I look out over the Derwent River. It's a fantastic view across the slate-grey sea. But below the tranquil surface of the Derwent, there is a battle raging between the locals and some aggressive new arrivals - northern Pacific seastars. The first of these seastars probably arrived as stowaways on ships from Japan about 20 years ago. They thrived in the cool Tasmanian waters and bred so furiously that there are now almost 30 million of them in the estuary.

The seastars could not have made the trip unaided because currents do not carry them this way. They were brought here in ships. The young seastars, swimming near the surface, were probably sucked up with tonnes of water, used as ballast to stabilise empty vessels. A single vessel can take 70,000 tonnes of ballast water - enough to fill 32 Olympic swimming pools and carry millions of young seastars.

The seastars would not have been alone - each day, around 3000 marine organisms are transported around the globe in the ballast water of ocean-going vessels. These invaders, along with the ballast water, are dumped at their destination to make way for precious cargo. In 1993, 121 million tonnes of ballast water were dumped into Australian ports - equal to one quarter the volume of water in Sydney Harbour. That's a lot of stowaways. Many of them will not survive the voyage, but those that do can settle in their new homes and, if conditions are right (as they were for the northern Pacific seastars), form plagues.

Seastars underwater in Tasmania's Derwent River

The northern Pacific seastars flourished because they left behind their natural predators, parasites and competitors. And they aren't fussy eaters. In 1993, during a community dive to try to rid the estuary of seastars, they were found feeding on a drowned sailor. The seastars usually eat more mundane fare - sea squirts, crustaceans, sponges and even other seastars. But they have huge appetites and prefer shellfish, particularly oysters and mussels. In 1954, they ate 400 million yen worth of marketable shellfish in Tokyo Bay. Because of their preference for commercial shellfish, the arrival of northern Pacific seastars in Australia caused great concern. In Tasmania, these fears were largely unrealised because the seastars were not found near shellfish leases. But early in 1998, hundreds of young seastars were discovered on commercial mussel ropes in Port Phillip Bay, Victoria. These seastars could have devastated the mussel crop. According to genetic tests, the northern Pacific seastars found in Victoria probably came from Tasmania. Not surprising, considering that in 1995, the numbers of seastar young in the Derwent estuary were the highest found anywhere in the world (up to 1100 per cubic metre of water). Young seastars were probably sucked up with ballast water from the Derwent estuary and later dumped into Port Phillip Bay.

The need to reduce the numbers of northern Pacific seastars in Australian waters and stop their spread is urgent. At the moment, the seastar is only found in Tasmanian and Victorian waters but it could spread along most of the southern Australian coast from Albany to Eden. Given its ravenous appetite and broad diet, it would force local communities and commercial shellfish in these waters to fight for their survival.

To stop the spread of seastars between Australian ports, scientists are calculating the best time and place to collect ballast water, to minimise the uptake of young seastars. Shellfish farmers also need to be vigilant because they often move ropes and trays used for growing mussels and oysters between bays and estuaries to improve condition of the shellfish. Young seastars spend the first three months of their lives swimming before they finally settle to the sea floor as microscopic copies of their parents. But they can also settle onto mussel ropes and oyster trays. The trays could be submerged briefly in freshwater to kill the seastars (it would not harm the shellfish) but this is not yet common practise among shellfish farmers.

Internationally, there are attempts to 'sterilise' ballast water so stowaways are not transported around the globe. It would be impractical to use chemicals because of the huge volume of ballast water carried on a single vessel. Ballast water systems are also being modified to prevent uptake (e.g. screens) or kill (e.g. heating, ozonation) stowaways in transit. In Australia, BHP modified its vessel, Iron Whyalla, to pump ballast waters through the engine to raise temperatures and kill seastars. Although these trials are promising, none of them are being used commercially. A more practical solution may be to exchange ballast waters, which are collected in foreign ports, in the open ocean. Marine organisms from open oceanic waters probably won't survive if they are dumped into coastal bays and harbours. To protect Australian harbours, the Australian Quarantine and Inspection Service (AQIS) is urging vessels to undertake voluntary ballast water management protocols, including exchanging ballast waters in the open ocean, when it is safe to do so.

Community divers pulled 30,000 sea stars from the Derwent River in 1993 and hardly put a dent in the population

But these measures will not reduce the numbers of seastars already in Australian waters. In 1993, community dives pulled 30,000 seastars from the Derwent but this hardly made a dent in the population. To protect commercial shellfish leases, specially designed traps (made in Tasmania) are used. They are similar to lobster traps and are baited with fish scraps. But traps cannot catch all the seastars in Australian waters. Dredging could collect more seastars at once but would disturb natural marine plants and animals as well as resuspend heavy metals in the sediments. Disposing of large numbers of seastars carcasses would also be a problem. Seastars can be composted and used as mulch but 30 million seastar corpses would stretch even the largest compost facility. So, collecting seastars may be useful in sensitive areas (like marine farms or marine reserves) but is probably not practical or possible for all of the northern Pacific seastars in Australian waters.

A financial reward would provide an incentive to hunt the seastars. In China, seastars are sold in markets for about $1US each. I am told they are delicious when steamed, but I doubt northern Pacific seastars will become a delicacy in Australia in the near future. Nor will pharmaceutical companies be harvesting the seastars because no bioactive compounds have been found in them. The northern Pacific seastar could replace seastars pulled from overseas reefs and then dried, painted and sold in interior decorating shops in Australia. This would help to protect seastars overseas as well as providing a cottage industry in Australia. At present, however, there is little financial incentive for people to hunt northern Pacific seastars in Australian waters.

Poisons are used against feral animals like foxes and cats. The problem with available seastar poisons (e.g. quicklime) is that they are not specific and, in the ocean, could damage natural marine communities. Also, the amount of chemicals needed to poison seastars in estuaries would be very expensive (and impractical). If poison baits could be designed which are attractive to seastars (and not local animals) they could be placed strategically to reduce seastar numbers. But as yet, no such bait has been found.

Seastars underwater in Tasmania's Derwent River

Parasites, on the other hand, tend to search specifically for seastars and attack them. In Australia, northern Pacific seastars don't have parasites (which probably allows them to flourish). In Japan, northern Pacific seastars are attacked by a tiny single-celled animal that looks like a hairy cucumber, called Orchitophrya. The name stems from the Greek and means Orchito - testes, phrya - loving. True to its name, Orchitophrya invades the testes, eats sperm and castrates the seastar. Scientists hoped this parasites would control northern Pacific seastars in Australia - the perfect biological control agent.

BUTů seastars have 10 testes and the parasite does not usually invade all of them. That means infected male seastars could still produce millions of sperm to fertilise eggs. Therefore, it's unlikely Orchitophrya will have the effect scientists had hoped for. A much more savage parasite is needed for the fight. In March 1999, scientists from CSIRO head to Korea to continue the search.

In the meantime, the search for a solution may be a lot closer to home. Some scientists believe that disturbed environments are more prone to invasion. In harbours and ports, dredging to improve shipping channels can destroy native marine communities that are already weakened by pollution. The building of wharves, piers and pylons gives arriving stowaways a vacant home as soon as they jump ship. In Tasmania, a new project (funded by the Natural Heritage Trust) will investigate the link between disturbed environments and the survival of the seastars. It will be interesting to see whether the best method to manage northern Pacific seastars in Australian waters may be to reduce pollution and enhance local plants and animals in estuaries.

In the future, molecular biology may be harnessed to attack the seastars. Scientists could insert or change a gene in northern Pacific seastars which would castrate them or kill their young. This gene could be delivered in a bait or by a parasite. For now, it is important to learn as much as possible about the seastar in Australian waters - its feeding, reproduction, predators and movements. This research will hopefully locate an Achilles heel that can be targeted with these new weapons.